Method for the thermal treatment of raw materials and a device for carrying out said method

ABSTRACT

The invention relates to a method for the thermal treatment of raw materials in which a furnace ( 2 ) is charged with the raw material and at least two gas phases are introduced into the furnace in which the raw material is subjected to the thermal treatment. The gas phases have different oxygen contents and are introduced into the furnace sequentially. During the introduction of a first gas phase with a relatively high oxygen content (for example, O 2 ), a temperature and pressure rise occurs; the reaction gases are forced towards the center of the furnace by the subsequent introduction of a second gas phase with a relatively low or no oxygen content (for example, air, CH 4 ), whereupon an equalization of temperature and concentration occurs in the melting zone. Said method permits a good penetration of the furnace charge and avoids localized overheating within the charge. The invention further relates to a furnace ( 2 ) for carrying out said method.

The present invention relates to a method for the thermal treatment of raw materials and to a device for carrying out this method for the thermal treatment of raw materials.

The various known shaft furnaces employing prior art technology have a great variety of industrial uses. For example, shaft furnaces can be used as melting units (e.g., blast furnaces, cupola furnaces) or for the heat treatment of a charge (e.g., lime kilns, calcining kilns for bauxite).

One method for operating a cupola furnace is known, for example, from DE 41 22 381 A1. Here, the cupola furnace is loaded at the top with the charge, a mixture of lumpy raw materials such as, for example, ore, scrap, coke charge, and fluxing materials. Through the injection of air (“blast”), the coke is oxidized to CO₂ and CO and thus produces the energy needed for melting. In this process, the air is conveyed into the furnace via tuyères, which are uniformly distributed in an annular plane on the circumference of the furnace. The molten pig iron is collected in the lower zone of the furnace, in what is called the hearth or hearth zone, and flows out continuously through a siphon system. In older cupola furnaces, air at room temperature (cold blast) is used for the partial combustion of the coke charge. Newer units are run with a hot blast, which is air regeneratively preheated with the offgas (blast furnace gas) in hot blast stoves. A coal gas, namely, CO₂ or CO, is introduced into the furnace according to this known method in order to maintain the CO content inside the furnace at a preset value or in order to vary the CO content inside the furnace.

Oxygen is added to the cold or hot blast in contemporary methods in order to increase the melting capacity and improve the metallurgical properties of the pig iron. Various advantages are achieved by the addition of oxygen. These include, among others, an increase in the temperature of the melting zone and a concomitant increase in melting capacity. Furthermore, higher pig iron temperatures are obtained, the coke charge burns more thoroughly (free residual coke can be found in the slag in the case of incomplete combustion), and less coke is therefore consumed. Of course, when oxygen is added, it must be noted that the pressure of the oxygen decreases within a short distance beyond the inlet with the result that a deeper penetration of the charge is not possible. Rather, a sharp temperature gradient occurs between the interior of the charge and the portion of the charge adjacent to the inlet. As a solution to this problem, DE 197 29 624 A1 proposes that the oxygen be introduced in high-pressure pulses into the furnace by means of a lance disposed in the tuyère. In this way the oxygen should penetrate the charge more deeply and thus eliminate the temperature gradient within the charge. With the high pressure and thus the higher energy, the known method does indeed allow a deeper penetration of the charge, but the results remain unsatisfactory. One of the reasons for this is that the large increase in the local reaction temperature in the vicinity of the O₂ inlet causes a superproportional, exponential increase in reaction rates. According to known formulas (e.g., the Arrhenius' equation), it can be assumed that, in the present case, the reaction rate doubles for each 20° C. to 30° C. increase in temperature.

The object of the present invention is to provide a method for the thermal treatment of raw materials that ensures the best possible penetration of the furnace charge and that avoids localized overheating within the charge. An additional goal is to introduce a device for carrying out the inventive method.

These objects are achieved by the characteristic features of patent claims 1 and 10, respectively. Advantageous embodiments of the invention are the subject matter of the dependent claims.

A furnace, such as a blast furnace, is used in the inventive method for the thermal treatment of raw materials. The raw materials are understood to be, for example, ore, scrap, coke charge, fluxing materials, etc. The furnace is charged with the raw materials and at least two gas phases are introduced into the furnace. The gas phases have different oxygen contents, and even an oxygen-free gas phase is permissible. According to the invention, the at least two gas phases are alternately introduced into the furnace. Here, alternating introduction is understood to mean that for a certain period of time only one of the gas phases is introduced, without the gas phases having been mixed in advanced, and that the gas phases can be alternately introduced in any sequence. It is possible in principle to introduce the gas phases with different oxygen contents by turns in a continuous volume flow or discontinuously, i.e., for a certain period of time neither the one nor the other gas phase is supplied.

In conventional methods the further penetration of the gas phase into the charge is blocked or, in unfavorable cases, the flow direction is even reversed. This is attributable to the fact that the oxidation products are predominantly gaseous, and the reactions are exothermic, with the result that sudden sharp increases in temperature and pressure occur locally.

This disadvantage is surmounted by the alternating introduction of gas phases with different oxygen contents. The aforementioned increases in temperature and pressure occur during the introduction of a first gas phase with a relatively high oxygen content (e.g., O₂); the reaction gases are forced back toward the center of the furnace by the subsequent introduction of a second gas phase with a relatively low or no oxygen content (e.g., air, CH₄), whereupon the temperature and concentration are equalized. It is particularly advantageous here that the equalizing reactions occur in deeper zones of the charge through increasingly intensive intermixing of the successive gases, wherein gas-gas reactions take place at significantly higher reaction rates. The sequencing of high oxygen content gas phases with low oxygen content gas phases has the advantage that a “priming charge” occurs with the high-oxygen gas phase at exponentially increased reaction rates and material conversions (especially in the gas-solid reaction with the coke charge), while the atmospheres and temperatures are equalized by the low-oxygen gas phase at high flow rates and thus good removal of the gaseous reaction products. In this way the inventive method allows good penetration of the furnace charge and avoids localized overheating within the charge.

In the inventive method, a high oxygen content gas phase, for example, pure oxygen, is thus briefly injected, preferably at high pressure, and causes a spontaneous oxidation. A low oxygen content gas phase, for example, compressed air, is then injected. The injections of oxygen and compressed air can overlap each other, follow one another immediately, or be separated by a time delay.

The method has the advantage that use of an oxygen-enriched air feed avoids the hindrance of coke oxidation by a “nitrogen” ballast. Through the induction of compressed air the local atmosphere is diluted and partially cooled with the result that the reaction equilibria are re-established. The equalization of the concentrations and temperatures through turbulence and diffusion owing to the batchwise introduction of oxygen and air accrues distinct advantages with regard to the execution of the reactions. In principle the charge serves as an internal mixing apparatus for the production of enriched air. Mixing can be rapidly adapted to the process by varying the different cycle lengths, with the result that fluctuations in the melt can be minimized.

In a preferred embodiment of the inventive method the at least two gas phases undergoing alternating introduction into the furnace are introduced into the furnace under a pressure that is greater than the internal pressure of the furnace. This greater pressure achieves a deeper penetration of the furnace charge, on the one hand, while, on the other hand, minimizing the influence of the wall effect in this zone. This can substantially improve the technical combustion efficiency and the metallurgical results.

In another preferred embodiment of the method, the at least two gas phases are introduced into the furnace in a continuous volume flow. Accordingly, there is no interruption between the introductions of the different gas phases, and as a result the gas phases immediately following one another drive each other and thus promote constant flow conditions. This results in good penetration of the charge.

More advantageously, one of the two gas phases in a preferred embodiment of the method is a mixture of an inert gas and oxygen. The inert gas is preferably N₂ and/or CO₂ and/or Ar.

One of the at least two gas phases is air in an advantageous embodiment of the inventive method.

One of the at least two gas phases is oxygen in another advantageous embodiment of the inventive method.

In order to be able to respond to changes in the operating parameters, in a preferred embodiment the proportions of the oxygen quantities introduced into the furnace with the at least two gas phases are regulated in order to establish a certain atmosphere in the furnace. This enables short-term and very sensitive responses to changes in the operating parameters.

In a preferred embodiment of the invention, the duration of introduction of the individual gas phases into the furnace is varied in order to adjust the quantities of oxygen delivered into the furnace with the at least two gas phases.

The proportions of the oxygen quantities introduced into the furnace with the two gas phases can be regulated in order to ensure that the atmosphere inside the furnace is always optimal.

The inventive furnace for the thermal treatment of raw materials has a chamber with a charging opening for the raw materials. This furnace is also provided with an injection apparatus for introducing the at least two gas phases with different oxygen contents into the chamber. The injection apparatus is designed in such a way that the at least two gas phases are introduced by turns into the furnace.

In an advantageous embodiment of the inventive furnace the injection apparatus for delivering the gas phases has gas phase conduits equipped with control valves. Gas phase conduit is understood to mean any mode of delivery by which the gas phases are conveyed.

The gas phases are introduced in sequences that can be arbitrarily varied in terms of length, the time interval between injections, and the relationship of the two gas phases to each other. The introduction can be regulated by means of stored-program control (SPC), e.g., with a microprocessor. In this manner the aforementioned parameters can be programmed variably and freely, and thus the inflow of the gas phases through the valves can be controlled.

In a preferred embodiment of the furnace according to the invention, pressure regulators are provided for adjusting the gas pressure in the gas phase conduits.

In order to allow the furnace to be monitored during operation and to allow the atmosphere to be changed, if need be, the furnace can be equipped with an analyzing device for analyzing the atmosphere in the furnace and/or a measuring device for measuring the temperature in the material discharged from the furnace.

In a particularly preferred embodiment, the inventive furnace is provided with one or more bustle pipes arranged around the furnace. The supply lances are connected to these bustle pipes.

Exemplary embodiments of the invention will be explained in detail in the following and with reference to the attached figures.

The following are shown:

FIG. 1 a cutaway side view of a first embodiment of the inventive furnace and

FIG. 2 a cutaway side view of a second and third embodiment of the inventive furnace.

FIG. 1 shows a furnace 2, which in the embodiment under discussion is devised as a cupola furnace. This cupola furnace serves to melt pig iron continuously. The furnace 2 has a furnace housing 4 that encloses a chamber 6. Considered from top to bottom the chamber 6 has several zones in which various processes are carried out, namely, a charging zone 8, a preheating zone 10, a melting zone 12, a so-called blast zone 14, and a hearth zone 16, with the last being situated directly above the furnace floor 18. The furnace housing 4 has, at the charging zone, a lateral charging opening 20, which can also be situated above the charging zone 8. The charging opening is designed such that the raw materials to be treated (not shown), e.g., a mixture of pig iron, scrap, coke, lime, alloying constituents, and other fluxing materials, can be delivered to the chamber 6 through the opening. In addition, the furnace housing in the charging zone has a vent 22 through which the resulting blast-furnace gas can be conducted away.

The shaft furnace 2 has a slag taphole 24 and an iron taphole 26. The furnace 2 also has a first bustle pipe 28, which surrounds the furnace housing 4 in annular fashion and is supplied with air via an air-supply device that is not shown. Uniformly distributed around the furnace 2 are a plurality of tubular tuyèress 30, which on the one side are connected to the first bustle pipe 28 and on the other side lead into the chamber 6 at the blast zone 14. The depicted first embodiment of the furnace 2 is also equipped with a second bustle pipe 32, which surrounds the furnace housing in annular fashion. Connected to the second bustle pipe 32 are essentially tubular gas phase supply lances 34, the ends 36 of which lead into the chamber 6 at the blast zone 14. In the first embodiment each of the gas phase supply lances 34 is disposed substantially coaxially in a tuyères 30 such that the end 36 of the gas phase supply lance 34 is situated in the center of the mouth of the tuyères 30 in the region of the chamber 6.

A first conduit 42 for delivering a first gas phase and a second conduit 44 for delivering a second gas phase flow into a common gas phase conduit 38, to which the second bustle pipe 32 is connected. A first blast-pressure tank 46 is connected to the first gas phase conduit 24 [sic], and a second blast-pressure tank 48 is connected to the second gas phase conduit 42 [sic]. The blast-pressure tanks contain pressurized gas phases with different oxygen contents, and the pressure within the blast-pressure tanks 46 and 48 is higher than the pressure in the blast zone 14 of the chamber 6 of the furnace 2. In the example under consideration, it is assumed that the gas phase conduits 42 and 44 deliver pure oxygen (O₂) to the first blast-pressure tank 46 and air to the second blast-pressure tank 48. The blast-pressure tanks 46 and 48 are not absolutely necessary but serve only as vibration dampers.

In the first gas phase conduit 42, a first pressure regulator 50 is placed upstream from the first blast-pressure tank 46 and a first control valve 52 is placed downstream from the blast-pressure tank 46, while in the second gas phase conduit 44 a second pressure regulator 54 is placed upstream from the second blast-pressure tank 48 and a second control valve 56 is placed downstream from the blast-pressure tank.

Furthermore, in order to regulate the inflow of the gas phases the furnace has a control unit 40 that is connected via control circuits 60 to the first and second pressure regulators 50 and 54 and to the first and second control valves 52 and 56.

The control unit 40 controls the control valves in such a manner that the common gas phase conduit 38 and the first gas phase conduit 42 communicate with each other while the second gas phase conduit 44 is closed off, or the common gas phase conduit 38 and the second gas phase conduit 44 communicate with each other while the first gas phase conduit 42 is closed off. The control unit adjusts the pressure in the blast-pressure tanks by means of the pressure regulators 50 and 54.

The operation of the first embodiment of the inventive furnace, illustrated in FIG. 1, is described in the following.

The chamber 6 of the furnace 2 is charged through the charging opening 20 with the raw materials (not shown) which are to be thermally treated, wherein the raw materials in the chamber 6 constitute the so-called charge. In the present example the raw materials are a mixture of pig iron, scrap, coke, lime, alloying constituents, and other fluxing materials. During the course of the process the continuously supplied raw materials pass through the various zones of the chamber 6 successively from top to bottom. The raw materials agglomerate in the charging zone 8. In the preheating zone 10 the raw materials are preheated by the heat and reaction gases rising from the underlying zones The preheated raw materials then pass into the melting zone 12, in which the raw materials begin to melt due to the high temperatures. The partially melted raw materials next pass into the blast zone 14. In the blast zone 14 air is blown through the first bustle pipe 28 and the tuyèress 30 into the chamber 6. The air needed for combustion is blown into the blast zone 14 as unheated cold blast or as hot blast. The raw materials are completely melted in the blast zone 14 by the air thus supplied, and the melt passes into the hearth zone 16, in which, on the one side, the slag floating on the melt is carried off through the first opening 24 and, on the other side, the melt is led away through the second opening 26.

In order to achieve good penetration of the charge and to avoid localized overheating within the charge, at least two gas phases with different oxygen contents are alternately introduced into the blast zone 14 of the chamber 6 of the inventive furnace 2. The gas phases are introduced in the process via the first and second gas phase conduits 42 and 44, the common gas phase conduit 38, and the gas phase supply lances 34. The continual alternation between the gas phases thus supplied is controlled by the control unit 54 by means of the control valves 52 and 56 such that the pressurized oxygen-rich gas phase (O₂) and the pressurized oxygen-poor gas phase (air) are introduced sequentially into the blast zone 14.

The times during which the control valves are open or closed can be changed by the control unit 54. Furthermore, the pressure of each gas phase can be changed by the pressure regulators 50 and 54, respectively. To establish a certain atmosphere in the furnace, the proportions of the oxygen quantities introduced into the furnace with the two gas phases are regulated, and to adjust the oxygen quantities the duration of gas phase introduction is changed.

In the above described first embodiment of the inventive furnace, the gas phase supply lances 3 [sic] are suitably arranged within the tuyèress 30 (preferably in concentric fashion), as shown in FIG. 1. However, the gas phase supply lances 34 can be situated above the tuyèress so that the gas phases are introduced above the blast, as shown in FIG. 2. Other arrangements of the gas phase supply lances 34 are also possible. For example, the angle of inclination of the gas phase supply lances 34 with respect to the furnace axis or their depth of immersion into the chamber 6 of the furnace 2 can be varied.

Furthermore, it may be desirable to arrange individual gas phase supply lances 34 or groups of gas phase supply lances 34 at different heights.

FIG. 2/left and FIG. 2/right, respectively, show second and third embodiments of the inventive furnace 2. The second embodiment (FIG. 2/left) differs from the first embodiment in that the former does not have a second bustle pipe 32. Instead, the inventive furnace 2 has several segmented bustle pipes 70, of which only one is shown, and which are arranged in annular fashion around the furnace 2. There is preferably a total of four circumferentially distributed segmented bustle pipes. The segmented bustle pipes are connected to a first gas phase conduit 38 and have gas phase supply lances 34 that open into the chamber 6 of the furnace 2. The gas phases can be admitted to each segmented bustle pipe 70 independently of the others. The gas phases are preferably admitted to the pipes 70 in sequence so that the gas phases are introduced clockwise or counterclockwise from each side into the furnace. Refer to the description of FIG. 1 for the other parts of the furnace.

The third embodiment (FIG. 2/right) differs from the first embodiment in that the former has separate gas phase supply lances 34 having one or two valves 72 each, such that each gas phase supply lance 34 has its own gas phase supply. A stored-program control (SPC) system, for example, can be employed in this case, wherein valve disturbances or control errors can be detected with feedback sensors for pressure or flow, for example, and eliminated. 

1-13. (canceled) 14: A method for the thermal treatment of raw materials comprising: (i.) charging a furnace with the raw materials and at least two gas phases with different oxygen contents; (ii.) subjecting the raw materials to the thermal treatment, wherein the at least two gas phases are alternately introduced into the furnace. 15: The method according to claim 14, wherein the at least two gas phases are introduced into the furnace at a pressure that is greater than the internal pressure of the furnace. 16: The method according to claim 14, wherein the at least two gas phases are introduced in a continuous volume flow into the furnace. 17: The method according to claim 14, wherein one of the at least two gas phases is a mixture of an inert gas and oxygen. 18: The method according to claim 17, wherein the inert gas is selected from the group consisting of N₂, CO₂, and Ar. 19: The method according to claim 14, wherein one of the at least two gas phases is air. 20: The method according to claim 14, wherein one of the at least two gas phases is oxygen. 21: The method according to claim 14, wherein the proportions of the oxygen quantities which is introduced into the furnace with the at least two gas phases are regulated in the furnace in order to establish a certain atmosphere in the furnace. 22: The method according to claim 21, wherein the duration for which the individual gas phases are introduced into the furnace is varied in order to adjust the quantities of oxygen delivered to the furnace with the at least two gas phases. 23: Furnace for the thermal treatment of raw materials comprising a chamber (6) that has a charging opening (20) for the raw materials and an injection apparatus for delivering at least two gas phases with different oxygen contents into the chamber (6), wherein the injection apparatus is designed to introduce the at least two gas phases alternately into the furnace (2). claim 24: The furnace according to claim 23, wherein the injection apparatus has two gas phase conduits (42, 44), each with a control valve (52, 54), for delivering the two gas phases with different oxygen contents into the chamber (6). 25: The furnace according to claim 24, wherein pressure regulators (50, 54) for adjusting the pressure of the gas phases are provided in the gas phase conduits (42, 44). 26: The furnace according to claim 23, wherein the furnace is equipped with a plurality of segmented bustle pipes (70) disposed around the furnace (2), wherein each pipe has a plurality of gas phase supply lances (34) that open into the chamber (6), and in that the gas phases can be admitted to each segmented bustle pipe (70) independently of the others. 